Thermal switch



March 4, 1941.

E. W. GENT THERMAL SWITCH Filed March 4, 1957 INVENTOR E. M. GENT @QQM ATmRNEY Patented Mar. 4, 194i UNITED STATES PATENT, OFFICE TelephoneLaboratories,

Incorporated, New

York, N. Y., a corporation of New York I Application March 4, 1937,Serial No. 128,966

1. Claim.

This invention relates to thermal switches of the type in which theoperation of a bimetallic strip is effective to close electricalcontacts for controlling the operation of -a heating element as used forexample in a quartz crystal oscillator housing for compensating foratmospheric temperature variations.

In such thermal switches, however, it has been found that the distanceor air gap variations between the electrical contact carried by thebimetallic strip and its cooperating contact carried by the support dueto the expansion of the latter when submitted to a difierence intemperature greatly ail'ect the operation of the bimetallic strip andtherefore that of the heating element controlled thereby.

The object of the invention is to provide a thermal switch of the typeabove referred to which will be simple, cheap to manufacture and whichwill operate in a positive and accurate manner.

A feature of the invention resides in'the provision of a bimetallicstrip which is formed by welding a leaf of a material possessing amaximum coeflicient of expansion to a leaf of a material havingsubstantially zero coemcient of expansion whereby maximum operatingamplitude and sensitiveness of the bimetallic strip thus formed isobtained.

Another feature resides in the provision of a support which is made of amaterial having substantially zero coeflicient of expansion whereby thevariations in the temperature of this support do not substantiallyaflect the operation of its cooperating bimetallic strip.

' Still another feature resides in the provision of means formed withthe support interlocking with means formed with the strip for holdingthe latter and a contact carried thereby in. operatlve relation with anassociated contacting member carried by the support.

Other novel features and advantages of the invention will appear by thefollowing description and by the claim appended thereto, reference beingbad to the accompanying drawing in which Fig. 1 is a diagrammatic viewof the operating circuit of the thermal switch of the invention;

Fig. 2 is a side elevation view showing the easing and a number ofoperating parts with portions cut away;

Fig. 3 is a front elevation view showing the casing in section and anumber of operating parts with portions cut away; and

Fig. 4 is a top assembly view showing the casing in section.

In the drawing, HI indicates a base of insulating material formed with ashoulder portion ll provided for mounting a metallic plate I! inposition perpendicular to the base I, this plate being held securelyagainst the shoulder portion H by screws l3 and I4 shown in Figs. 2, 3and 4. A support l5 formed with an arcuated shape in the plane of itsthickness so as to obtain maximum rigidity for a minimum thickness insuch a plane is made of a material having substantially zero coefllcientof expansion such as a nickel steel alloy. This support is insulativelymounted on plate l2 and is secured thereon by screws [8 and I1. SupportI5 is provided at its ends across its thickness with hooked portions l8and I9 shown in Fig. 3 for engaging the V- shaped recesses l8 and I! atthe ends in a manner to position the width of the strip at right anglesto the plane of support I5 while the eileotive bimetallic strip of alength of the strip 20, that is, the length directly across the hooks I8and I9 oi the support i5 is slightly longer so as to cause this strip tonormally flex in the position as shown in Fig. 3, this strip being madeof a brass leaf and a leaf of nickel steel having substantially zerocoemcient of expansion.

An adjustable contact in the form of a screw 2! engages an insulatingbushing 22 in the support l5, this contact being normally engaged bycontact 23 carried by the metallic strip for completing an energizingcircuit to a heater element which will be hereinafter described indetail. On the metallic plate [2 and adjacent the bimetallic strip 20 ismounted the heater element consisting of an insulating strip 24 servingas a core for the resistor wire 25 coiled thereon, the strip 24 beingheld securely at each end on the plate l2 by two sets of screws 26-21and 2829 shown in Fig. 3.

To the opposite side of plate l2 are mounted the electrodes 3! and 28and a quartz crystal plate 38 shown in Figs. 2 and 4. The electrodes 31and 38 and the plate 39 are held firmly against the metallic plate 12 bya clamping spring ll having its lower end portion engaged in a slot iiin the base to and its upper disposed end portion engaging in a groovein the end of an insulating stud secured in the metallic plate l2.

The adjustable contact 2! is electrically connected to a plug 3 by awire 36 shown in Figs. 2, 3 and 4, this plug having one end embedded inthe base It. The resistor wire 25 at one end is electrically connectedto the bimetallic strip II and therefore to the support I! by a wire 3|while the other end of resistor wire I! is connected to a terminal Si bya wire 32. The terminal II is secured to the electrode 31 by a screw 56and one end of the resistor wire 25 and the electrode 31 connected to aplug II by a wire 5|, while the electrode ll is connected to a plug 52by a wire I, the plugs III and I! having each end embedded in the baseI. in position to form, in cooperation with plug 35, a triangularformation for engaging a group of similarly disposed connecting jacksnot shown.

The base I is formed with a peripheral shoulder portion 54 against whichthe edge of a casing 55 abuts to form therewith a dustproof closurebetween the base III and the casing 55, this casing being securely heldon the base II by screws 56 and 51 shown in Figs. 3 and 4.

In the operation or the thermal switch of the invention and supposingthe bimetallic strip in the position shown in Fig. 3, that is, withcontacts 2| and 23 closed which occurs upon a drop in the temperature,the current will flow from the positive side of battery '0 through wire36, the contacts 2| and 23, the bimetallic bar II, the wire ill. theresistor wire I! to the negative side of battery i0 through conductors"and II, thus energizing the heater element. The heat generated by saidelement heats the bimetallic bar 2. which upon reaching a suiiicientlyhigh temperature is caused, through the elongation of the brass elementin the bimetallic strip, to move in the position shown in dotted linesin Fig. 3, for breaking the heater element circuit and therebymaintaining the temperature in the housing 55 within a few degrees oftemperature difl'erence, which may be controlled by the ad- Justment ofthe contact 2|. .By the use 0! a material having a substantially zerocoeiflcient of expansion for the support I and for one o! the elementsof the bimetallic strip 20 added to the arcuated shape of the support I!and its consequent great rigidity, a thermostat device is obtained whichoperates with a high degree of accuracy and thereby makes possible tomaintain the operating temperature of the quartz crystal platesubstantially constant.

What is claimed is:

In a thermal switch, a base, a plate secured to said baseperpendicularly thereof, an oblongshaped bimetallic strip, a plate-likearcuate mounting of unit structure for said strip having a reenforcedportion midway betweenits ends and means formed at one 0! its ends forsecuring it to said plate in a plane parallel thereto, and projectionsat the ends of said mounting converging towards each other to form hookportions, and notches formed at each end of said strip for engaging saidhook. portions so as to position the plane of said mounting at rightangles to the width of said strip.

EDGAR W. GENT.

